Mosquitto will support the v3.1 spec in a future release, along with the
ability to control both broker and topic access by username. In the meantime,
if you need this functionality, the IBM proprietary RSMB broker may be
suitable for testing purposes. The RSMB package now also includes an MQTT
client library, a simple publish client and a simple subscribe client, just
like mosquitto. Be sure to check the license terms before using it!